High resolution optical-feedback cavity ring-down spectroscopy at 8.45 µm - Quentin Fournier (LAME, LIPhy)

Absorption spectroscopy has proven to be a very effective tool for a wide range of applications, from the physico-chemical study of the atmosphere to fundamental physics. In recent decades, advances in combining lasers with resonant optical cavities have led to unprecedented levels of absorption sensitivity and frequency resolution, especially in the near-infrared range. However, similar instruments for the mid-infrared are still rare, even though this spectral range is highly valuable for spectroscopic applications. This is mainly because suitable optical components are lacking at these wavelengths and their performance is generally lower, making it difficult to directly adapt cavity-based spectroscopy techniques.

The work presented in this thesis is primarily dedicated to the development of a spectrometer addressing these requirements for sensitivity and frequency resolution in the mid-infrared. Two prototypes have been developed, based on combining a quantum cascade laser (QCL) emitting at 8.45 µm and a high-finesse linear optical cavity. These instruments use a strategy that allows both cavity ring-down spectroscopy (CRDS) and laser stabilization through optical feedback from the same linear cavity (LOF), ensuring optimal photon injection.

The first system, referred to as LOF-CRDS, enables the acquisition of spectra over a frequency range of approximately 100 GHz, with a frequency resolution of 300 MHz. The very high absorption sensitivity of about 4×10⁻¹⁰ cm⁻¹, made it possible to measure the water vapor self-continuum, which is critical in atmospheric physics for modeling Earth’s radiative transfer.

A second system, named LOFS-CRDS, uses the same principles but is designed to detect ultra-narrow absorption features (sub-MHz frequency resolution). It achieves this by actively controlling the resonant cavity length. This system allowed the recording of spectra with a frequency resolution of 20 kHz, a four-order-of-magnitude improvement over the previous one, while reaching a limit of detection of 2×10⁻¹⁰ cm⁻¹. The technique’s performance and limitations were assessed through two case studies: one on the line profiles of two H₂O transitions, and another on a water Lamb dip, a sub-Doppler feature with a width near one megahertz, originating from an optical saturation effect.

Our results open many possibilities for the LOFS-CRDS instrument, especially for high-resolution and high-sensitivity spectroscopic applications, such as studying the rovibrational transitions of cold C60, which is crucial in astrophysics. As a key component, our instrument could allow for the quantitative measurement of its concentration in a supersonic jet, a prerequisite for studying its spectrum in other wavelength ranges. Furthermore, the LOF-CRDS technique could be integrated into field instruments because it is easy to implement, or serve as the core of more fundamental studies once its frequency axis is absolutely calibrated.